In an inductively coupled plasma (ICP) emission spectrometer, plasma is generated and maintained by introducing a plasma-generating gas such as argon as well as an analysis sample into a plasma torch and supplying high-frequency power to an induction coil. An elemental composition of the sample is analyzed by measuring a light emission from sample atoms excited in the plasma by using a spectroscope or the like.
A high-frequency electromagnetic field is produced in a plasma-generation portion by a high-frequency current flowing through the induction coil, and the plasma is heated by induction current caused by acceleration of charged particles in the plasma. Since this induction current decreases magnetic fields formed by the induction coil, effective inductance of the induction coil decreases. Moreover, energy loss due to heating of the plasma provides a resistance component of impedance of the induction coil.
While the plasma is formed, impedance of the induction coil varies. A state of the plasma varies according to the plasma-generating gas, condition of the analysis sample, supplied power to the plasma, etc., and the impedance of the induction coil also varies.
A resonant circuit is formed by the induction coil and a capacitor to supply power to the plasma. This resonant circuit is driven by a high-frequency power supply that provides high-frequency power, for example, in a range of several 100 W to several kW at 27 MHz. Since an output impedance of a high-frequency power supply is designed to be 50Ω in general, a tuning circuit is arranged between the high-frequency power supply and the resonant circuit and is controlled so that the impedance seen from the high-frequency power supply side is 50Ω all the time. It is a common method that a vacuum variable capacitor in the tuning circuit is adjusted by using a motor or the like so that the reflected power from the tuning circuit becomes zero.
In this way, it is relatively easy to design a highly efficient high-frequency power supply under conditions of a constant resonant frequency and a constant load impedance. However, in the case that the load impedance varies, maintaining the optimum condition all the time by operating the tuning circuit requires a complicated control mechanism and costly parts, which has become a negative factor in application to industrial products.
A free-running method with changing frequency in response to the variation in the load impedance without using costly parts such as the vacuum variable capacitor is, for example, disclosed in Patent Document 1. In Patent Document 1, the frequency of the high-frequency power supply is changed by using a voltage controlled oscillator (VCO) so that the reflected power from the tuning circuit is minimized. Nonetheless, since variable elements such as the vacuum variable capacitor or the like are removed from the tuning circuit, variation in the resistance component of the induction coil cannot be coped with.
Because of this, a method of directly driving the resonant circuit without limiting the output impedance of the high-frequency power supply to 50Ω is also disclosed. It is possible to provide a cheap power source with high efficiency although the high-frequency power supply cannot be placed far from the induction coil by using a transmission line such as a coaxial cable or the like, and the high-frequency power supply has to be placed near the induction coil. Due to employment of a self oscillation method of automatically changing the frequency in response to the variation of the load impedance, it is possible to construct a simpler high-frequency power supply circuit eliminating a frequency control circuit and a tuning circuit.
A self-oscillating high-frequency power supply circuit in an induction heating apparatus is disclosed in Patent Document 2 where a portion of voltage of the resonant circuit is fed back to a metal-oxide-semiconductor field-effect transistor (MOSFET) switching element in the high-frequency power supply. Moreover, a method of using a vacuum tube as the switching element is disclosed in Patent Document 3, and a method of using a transistor as the switching element is disclosed in Patent Document 4. Further, in Patent Document 5, a method of driving one side of the resonant circuit with a half bridge and a method of driving two sides of the resonant circuit with a full bridge are disclosed. In Patent Document 2, a portion of voltage of the capacitor in the resonant circuit is used as a gate voltage of a MOSFET by transformer coupling. In Patent Document 5, a current in the resonant circuit is fed to a resistor by transformer coupling for use as a gate voltage of the MOSFET.
Among these methods, even when variation occurs in the load impedance, oscillation continues at a resonant frequency determined by the load impedance, the switching element such as the MOSFET or the like are driven automatically at the same frequency due to feedback from the resonant circuit, and a process such as phase control between the resonant circuit and the drive circuit is not required any more.